Measurement of radon level in ambient air is becoming increasingly important. Exposures to high concentration of radon daughters have been epidemiologically correlated to an increased incidence of lung cancer in uranium miners. Recent indoor measurements indicate that radon daughter concentrations in a number of radon-contaminated dwellings are comparable to levels found in uranium mines. Radon daughter dosimeters which are precise and modestly priced are urgently needed for large-scale survey projects.
The main components of a typical radon daughter dosimeter include an aerosol sampling device and a detector. In order to determine the radon daughter level per unit volume of air, the aerosol sampling device is usually equipped with a suction pump and a filter system upon which the alpha-emitting particles are deposited. There are several versions of air samplers suitable for the radon daughters dosimeter. As an example, the aerosol sampling device of the personal dosimeter described by J. A. Auxler, et al entitled "A New Progeny Personal Dosimeter" and published in 1971 by Pergamon Press in Health Physics, Vol. 21, pages 126 to 128, is one of them.
A detecting system capable of distinguishing RaA and RaC' is described in the U.S. Pat. No. 3,922,555 (Chapuis and DaJlevic from Commissariat a l'Energie Atomique (CEA, Paris, France)). The system includes two collimators with filter paper at one end for retaining RaA and RaC'-laden aerosol and, at the other end, a nitrocellulose detector covered with two absorbers of different thicknesses. The optimum length for the air gap is 2.2 cm. Alphas emitted by RaA and RaC' passing through different absorbers are slowed down to the required level (3 MeV) and are detected in separate fields. Although the above patent is designed as a personal dosimeter for use in uranium mines, it can be modified to perform well in indoor as well as environmental surveys.
Experience in the field with the service of U.S. Pat. No. 3,922,555 has revealed the following problems:
(1) Uranium mine personal dosimeter--Contamination of the collimator system by long-life alphas from uranium dust--especially in the high-dust, uranium-rich mining environment, is well-known in used collimators. Recently, it was also found that a contamination problem exists for RaA even for a brand-new collimator. Latest investigation reveals that if uranium dust is allowed into the collimator, alphas from the uranium dust will have the same residual energy as alphas from RaA deposited on the filter paper. As a result, a false high measurement of RaA will be obtained. The problem can be solved either by
(a) limiting the passage of uranium dust into the collimator, or PA1 (b) changing the CEA geometry so that the air gap (2.2 cm) can be shortened and the thickness of the absorber can be increased.
Moreover, decontamination of a used collimator is a serious problem. One working solution is to design a collimator system which is disposable.
(2) Indoor Survey--Usually the purpose of the indoor survey is to estimate annual mean of WL for a given indoor environment. The so-called time-average instrument is designed to measure the mean diurnal value as well as variations introduced by living habits of the occupants. In order to obtain a reliable mean value, 10 days' continuous exposure is required. Latest findings on badly radon-contaminated houses present another urgent requirement for indoor radon surveys. The purpose of this survey, usually known as "screening", is to identify houses with high radon levels for remedial action or further investigation. The survey requires instruments with minimal processing time and therefore detecting heads with higher efficiency are preferred. Detecting head design for this purpose usually cannot be used for the ordinary time-average purpose described above, as overlapping of tracks will be found. A more versatile design is needed so that detecting geometry can be changed in a simple way.
(3) Environmental survey use--A low radon level in the environment requires a high-speed battery-operated pump to compensate for the low efficiency imposed by adopting collimators. The power supply required by the high-speed pump is usually not convenient for field use. It is also expensive.